Most existing Wireless Local Area Network (WLAN) systems based upon OFDM modulation techniques comply with the IEEE 802.11a/g standard (see, IEEE Std 802.11a-1999, “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specification: High-speed Physical Layer in the 5 GHz Band”). In order to support evolving applications, such as multiple high-definition television channels, WLAN systems must be able to support ever increasing data rates. Accordingly, next generation WLAN systems should provide increased robustness and capacity.
The OFDM modulation technique is especially sensitive to a DC offset in the received signal. Receiver DC offset is due to, among other things, self-mixing of the local oscillator in the radio circuit, leading to a constant term that is superimposed on the useful signal components going into the Analog-to-Digital (A/D) conversion and subsequently into the Digital Signal Processor (DSP). In particular, if there is a DC offset as well as residual frequency offsets in an OFDM system, the unwanted signal components can overlap at least partially on the lower OFDM subcarriers, leading to significant performance degradations.
A number of techniques have been proposed or suggested to mitigate or reduce the effects of DC offset (e.g., DC offset calibration or AC coupling). Nonetheless, it remains difficult to meet the requirements of the higher data rates of the OFDM modulation specification (such as 64 QAM at 54 Mbps or higher). One proposed technique attempts to avoid the DC offset entirely in the analog domain. It has been found, however, that it is impossible to always avoid DC offset, especially when there are temperature fluctuations. Existing analog domain DC compensation schemes are insufficient to provide the degree of “cleanliness” needed for high-speed modes in OFDM or MIMO-type of extensions. In the context of 802.11a/g, it has been suggested to avoid frequency offset in the analog domain. In such an implementation, the DC term falls on the zero-th subcarrier, which is not utilized in the standard specification. Nonetheless, it is often difficult to control the analog parameters to a degree required by high-speed modes in OFDM or MIMO-type of extensions.
U.S. patent application Ser. No. 10/918,223, entitled “Method and Apparatus for Estimating DC Offset in an Orthogonal Frequency Division Multiplexing System,” discloses techniques for estimating and compensating for DC offset in an OFDM receiver. Generally, the DC offset estimation is accomplished by subtracting a sum of time domain samples of an OFDM symbol for two consecutive OFDM symbols or subtracting a known transmitted OFDM symbol and a frequency domain representation of a received version of the known OFDM symbol (at least one of which is adjusted to compensate for channel distortion).
A need still exists for improved methods and systems to estimate the DC offset in an OFDM receiver. A further need exists for improved methods and systems that compensate for such DC offset.